385 research outputs found

    Pulsed Free Space Photonic Vector Network Analyzers

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    Terahertz (THz) radiation (0.1–10 THz) has demonstrated great significance in a wide range of interdisciplinary applications due to its unique properties such as the capacity to penetrate optically opaque materials without ionizing effect, superior spatial resolution as compared to the microwave domain for imaging or ability to identify a vast array of molecules using THz fingerprinting. Advancements in generation and detection techniques, as well as the necessities of application-driven research and industry, have created a substantial demand for THz-range devices and components. However, progress in the development of THz components is hampered by a lack of efficient and affordable characterization systems, resulting in limited development in THz science and technology. Vector Network Analyzers (VNAs) are highly sophisticated well-established characterization instruments in the microwave bands, which are now employed in the lower end of the THz spectrum (up to 1.5 THz) using frequency extender modules. These modules are extremely expensive, and due to the implementation of hollow metallic waveguides for their configuration, they are narrowband, requiring at least six modules to achieve a frequency coverage of 0.2–1.5 THz. Moreover, they are susceptible to problems like material losses, manufacturing and alignment tolerances etc., making them less than ideal for fast, broadband investigation. The main objective of this thesis is to design a robust but cost-effective characterization system based on a photonic method that can characterize THz components up to several THz in a single configuration. To achieve this, we design architectures for the Photonic Vector Network Analyzer (PVNA) concept, incorporating ErAs:In(Al)GaAs-based photoconductive sources and ErAs:InGaAs-based photoconductive receivers, driven with a femtosecond pulsed laser operating at 1550 nm. The broadband photonic devices replace narrowband electronic ones in order to record the Scattering (S)-parameters in a free space configuration. Corresponding calibration and data evaluation methods are also developed. Then the PVNAs are configured, and their capabilities are validated by characterizing various THz components, including a THz isolator, a distributed Bragg Reflector, a Split-Ring Resonator array and a Crossed-Dipole Resonator (CDR) array, in terms of their S-parameters. The PVNAs are also implemented to determine the complex refractive index or dielectric permittivity and physical thickness of several materials in the THz range. Finally, we develop an ErAs:In(Al)GaAs-based THz transceiver and implement it in a PVNA configuration, resulting in a more compact setup that is useful for industrial applications. The feasibility of such systems is also verified by characterizing several THz components. The configured systems achieve a bandwidth of more than 2.5 THz, exceeding the maximum attainable frequency of the commercial Electronic Vector Network Analyzer (EVNA) extender modules. For the 1.1-1.5 THz band, the dynamic range of 47-35 dB (Equivalent Noise Bandwidth (ENBW) = 9.196 Hz) achieved with the PVNA is comparable to the dynamic range of 45-25 dB (ENBW = 10 Hz) of the EVNA. Both amplitude and phase of the S-parameters, determined by the configured PVNAs, are compared with simulations or theoretical models and showed excellent agreement. The PVNA could discern multi-peak and narrow resonance characteristics despite its lower spectral resolution (∌3-7 GHz) compared to the EVNA. By accurately determining the S-parameters of multiple THz components, the transceiver-based PVNA also demonstrated its exceptional competence. With huge bandwidth and simpler calibration techniques, the PVNA provides a potential solution to bridge the existing technological gap in THz-range characterization systems and offers a solid platform for THz component development, paving the way for more widespread application of THz technologies in research and industry

    Recent progress of photodetector based on carbon nanotube film and application in optoelectronic integration

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    Due to its remarkable electrical and optical capabilities, optoelectronic devices based on the semiconducting single-walled carbon nanotube (s-SWCNT) have been studied extensively in the last two decades. First, s-SWCNT is a direct bandgap semiconductor with a high infrared absorption coefficient and high electron/hole mobility. In addition, as a typical one-dimensional material, there is no lattice mismatch between s-SWCNT and any substrates. Another advantage is that the optoelectronic devices of s-SWCNT can be processed at low temperatures. s-SWCNT has intriguing potential and applications in solar cells, light-emitting diodes (LEDs), photodetectors, and three-dimensional (3D) optoelectronic integration. In recent years, along with the advancement of solution purification technology, the high-purity s-SWCNTs film has laid the foundation for constructing large-area, homogenous, and high-performance optoelectronic devices. In this review, optoelectronic devices based on s-SWCNTs film and related topics are reviewed, including the preparation of high purity s-SWCNTs film, the progress of photodetectors based on the s-SWCNTs film, and challenges of s-SWCNTs film photodetectors

    Integrated widely tunable laser systems at 1300 and 1550 nm as swept sources for optical coherence tomography

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    Near-infrared photodetection in nanocarbon materials

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    The conversion of light into electricity is at the heart of solar cells and photodetectors and in other optoelectronic devices used in telecommunication systems. In particular, near-infrared (NIR) photodetection is very relevant for applications in night vision, remote sensing, food inspection, and surveillance. Novel materials that enable broadband NIR photodetection are sought, and the emerging class of nanocarbon and other 2D materials hold promise for large device photoresponsivities, high-speed detection, spectral control of the photoresponse, ease of integration, and waferscale fabrication. In this thesis, two types of nanocarbon materials have been explored for broadband NIR photodetection: nanocrystalline graphene (NCG) and single-walled carbon nanotubes (SWCNTs) networks. Graphene is a gapless 2D semi-metal with wavelength-independent light absorption with only 2.3% of the incident photons in a wide wavelength range. The growth of multi-layer graphene with predefined thickness for increased absorption has not yet been realized. To this end, nanocrystalline graphite (NCG), synthesized with a defined thickness on a silicon wafer, is introduced as a material for near-infrared to short-wavelength infrared (SWIR) photodetection. A broadband spectrally flat photoresponse was obtained in the NIR-SWIR spectral region, and the detected photocurrents were attributed to a temperature-induced bolometric effect. The SWCNT networks with a diameter distribution tailored for the near-infrared photodetection are grown using chemical vapor deposition (CVD) process on SiO2/p-Si substrates. The SWCNT networks are complementarily characterized using multi-wavelength resonant Raman spectroscopy and scanning photocurrent spectroscopy. The photocurrent data confirms a broadband optical response to the near-infrared light indicating a large diameter distribution in the CNT network. Devices are fabricated in a transistor geometry to study the spatial photoresponse distribution under different biasing schemes in the 1100 nm – 1800 nm spectral region, and the resulting photoresponse is discussed in terms of the photodetection mechanisms. During the course of this thesis, the photocurrent data were obtained with an in-house developed aberration-corrected scanning photocurrent setup. In order to enhance light-matter interaction in nanocarbon materials, the so-called plasmonic-photonic (PPhC) structures with optical resonances in visible-nIR spectrum were fabricated and characterized to investigate Raman enhancement in graphene. The local enhancements in the PPhCs were understood from the complementary near-field and far-field simulations optical simulations

    Charge-carrier dynamics in organic LEDs

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    Anyone who decides to buy a new mobile phone today is likely to buy a screen made from organic light-emitting diodes (OLEDs). OLEDs are a relatively new display technology and will probably account for the largest market share in the upcoming years. This is due to their brilliant colors, high achievable display resolution, and comparably simple processing. Since they are not based on the rigid crystal structure of classical semiconductors and can be produced as planar thin-film modules, they also enable the fabrication of large-area lamps on flexible substrates – an attractive scenario for future lighting systems. Despite these promising properties, the breakthrough of OLED lighting technology is still pending and requires further research. The charge-carrier dynamics in an OLED determine its device functionality and, therefore, enable the understanding of fundamental physical concepts and phenomena. From the description of charge-carrier dynamics, this work derives experimental methods and device concepts to optimize the efficiency and stability of OLEDs. OLEDs feature an electric current of charge carriers (electrons and holes) that are intended to recombine under the emission of light. This process is preceded by charge-carrier injection and their transport to the emission layer. These three aspects are discussed together in this work. First, a method is presented that quantifies injection resistances using a simple experiment. It provides a valuable opportunity to better understand and optimize injection layers. Subsequently, the charge carrier transport at high electrical currents, as required for OLEDs as bright lighting elements, will be investigated. Here, electro-thermal effects are presented that form physical limits for the design and function of OLED modules and explain their sudden failure. Finally, the dynamics and recombination of electro-statically bound charge carrier pairs, so-called excitons, are examined. Various options are presented for manipulating exciton dynamics in such a way that the emission behavior of the OLED can be adjusted according to specific requirements.:List of publications . . . . . . . . . . . . . . . . . v List of abbreviations . . . . . . . . . . . . . . . . . ix 1 Introduction . . . . . . . . . . . . . . . . . 1 2 Fundamentals . . . . . . . . . . . . . . . . . 5 2.1 Light sources and the human society . . . . . . . . . . . . . . . . . 5 2.1.1 Human light perception . . . . . . . . . . . . . . . . . . . . 8 2.1.2 Physical light quantification . . . . . . . . . . . . . . . . . . 10 2.1.3 Non-visual light impact . . . . . . . . . . . . . . . . . . . . . 13 2.1.4 Implications for modern light sources . . . . . . . . . . . . . 15 2.2 Organic semiconductors . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2.1 Molecular energy states . . . . . . . . . . . . . . . . . . . . . 18 2.2.2 Intramolecular state transitions . . . . . . . . . . . . . . . . 24 2.2.3 Molecular films . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.2.4 Electrical doping . . . . . . . . . . . . . . . . . . . . . . . . 34 2.2.5 Charge-carrier transport . . . . . . . . . . . . . . . . . . . . 36 2.2.6 Exciton formation and recombination . . . . . . . . . . . . . 38 2.2.7 Exciton transfer . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.3 Organic light-emitting diodes . . . . . . . . . . . . . . . . . . . . . 44 2.3.1 Structure and operation principle . . . . . . . . . . . . . . . 44 2.3.2 Metal-semiconductor interfaces . . . . . . . . . . . . . . . . 47 2.3.3 Typical operation characteristics . . . . . . . . . . . . . . . . 49 2.4 Colloidal nanocrystal emitters . . . . . . . . . . . . . . . . . . . . . 52 2.4.1 Terminology: Nanocrystals and quantum dots . . . . . . . . 52 2.4.2 The particle-in-a-box model . . . . . . . . . . . . . . . . . . 54 2.4.3 Surface passivation . . . . . . . . . . . . . . . . . . . . . . . 55 3 Materials and methods . . . . . . . . . . . . . . . . . 57 3.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.1 OLED materials . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.2 Materials for photoluminescence . . . . . . . . . . . . . . . . 60 3.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.2.1 Thermal evaporation . . . . . . . . . . . . . . . . . . . . . . 62 3.2.2 Solution processing . . . . . . . . . . . . . . . . . . . . . . . 64 3.3 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.3.1 Absorbance spectroscopy . . . . . . . . . . . . . . . . . . . . 66 3.3.2 Photoluminescence quantum yield . . . . . . . . . . . . . . . 66 3.3.3 Excitation sources . . . . . . . . . . . . . . . . . . . . . . . 67 3.3.4 Sensitive EQE for absorber materials . . . . . . . . . . . . . 68 3.4 Exciton-lifetime analysis . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.1 Triplet lifetime . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.2 Singlet-state lifetime . . . . . . . . . . . . . . . . . . . . . . 70 3.4.3 Lifetime extraction . . . . . . . . . . . . . . . . . . . . . . . 70 3.5 OLED characterization . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.5.1 Current-voltage-luminance and quantum efficiency . . . . . . 73 3.5.2 Temperature-controlled evaluation . . . . . . . . . . . . . . . 74 4 Charge-carrier injection into doped organic films . . . . . . . . . . . . . . . . . 77 4.1 Ohmic injection contacts . . . . . . . . . . . . . . . . . . . . . . . . 79 4.2 Device architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.1 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.2 Device symmetry . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.3 Device homogeneity . . . . . . . . . . . . . . . . . . . . . . . 83 4.3 Resistance characteristics . . . . . . . . . . . . . . . . . . . . . . . . 84 4.3.1 Experimental results . . . . . . . . . . . . . . . . . . . . . . 84 4.3.2 Equivalent-circuit development . . . . . . . . . . . . . . . . 85 4.4 Impedance spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 92 4.4.1 Measurement fundamentals . . . . . . . . . . . . . . . . . . 92 4.4.2 Thickness dependence . . . . . . . . . . . . . . . . . . . . . 93 4.4.3 Temperature dependence . . . . . . . . . . . . . . . . . . . . 95 4.5 Depletion zone variation . . . . . . . . . . . . . . . . . . . . . . . . 97 4.6 Molybdenum oxide as a case study . . . . . . . . . . . . . . . . . . 99 5 Charge-carrier transport and self-heating in OLED lighting . . . . . . . . . . . . . . . . .101 5.1 Joule self-heating in OLEDs . . . . . . . . . . . . . . . . . . . . . . 104 5.1.1 Electrothermal feedback . . . . . . . . . . . . . . . . . . . . 104 5.1.2 Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 5.1.3 Cooling strategies . . . . . . . . . . . . . . . . . . . . . . . . 106 5.2 Self-heating causes lateral luminance inhomogeneities in OLEDs . . 108 5.2.1 The influence of transparent electrodes . . . . . . . . . . . . 108 5.2.2 Luminance inhomogeneities in large OLED panels . . . . . . 110 5.3 Electrothermal OLED models . . . . . . . . . . . . . . . . . . . . . 112 5.3.1 Perceiving an OLED as thermistor array . . . . . . . . . . . 112 5.3.2 The OLED as a single three-layer thermistor . . . . . . . . . 114 5.3.3 A numerical 3D model of heat and current flow . . . . . . . 116 5.4 OLED stack and experimental conception . . . . . . . . . . . . . . 118 5.5 The Switch-back effect in planar light sources . . . . . . . . . . . . 120 5.5.1 Predictions from numerical 3D modeling . . . . . . . . . . . 121 5.5.2 Experimental proof . . . . . . . . . . . . . . . . . . . . . . . 124 5.5.3 Variation of vertical heat flux . . . . . . . . . . . . . . . . . 127 5.5.4 Variation of the OLED area . . . . . . . . . . . . . . . . . . 131 5.6 Electrothermal tristabilities in OLEDs . . . . . . . . . . . . . . . . 133 5.6.1 Observing different burn-in schematics . . . . . . . . . . . . 133 5.6.2 Bistability and tristability in organic semiconductors . . . . 134 5.6.3 Experimental indications for attempted branch hopping . . . 138 5.6.4 Saving bright OLEDs from burning in . . . . . . . . . . . . 144 5.6.5 Taking another view onto the camera pictures . . . . . . . . 145 6 Charge-carrier recombination and exciton management . . . . . . . . . . . . . . . . .147 6.1 Optical down conversion . . . . . . . . . . . . . . . . . . . . . . . . 149 6.1.1 Spectral reshaping of visible OLEDs . . . . . . . . . . . . . 149 6.1.2 Infrared-emitting OLEDs . . . . . . . . . . . . . . . . . . . . 155 6.2 Dual-state Förster transfer . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.2 Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 6.3 Singlet fission and triplet fusion in rubrene . . . . . . . . . . . . . . 161 6.3.1 Photoluminescence of pure and doped rubrene films . . . . . 163 6.3.2 Electroluminescence transients of rubrene OLEDs . . . . . . 172 6.4 Charge transfer-state tuning by electric fields . . . . . . . . . . . . . 177 6.4.1 CT-state tuning via doping variation . . . . . . . . . . . . . 177 6.4.2 CT-state tuning via voltage . . . . . . . . . . . . . . . . . . 180 6.5 Excursus: Exciton-spin mixing for wavelength identification . . . . 183 6.5.1 Characteristics of the active film . . . . . . . . . . . . . . . . 184 6.5.2 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 6.5.3 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 6.5.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 6.5.5 Application demonstrations . . . . . . . . . . . . . . . . . . 192 6.5.6 All-organic device . . . . . . . . . . . . . . . . . . . . . . . . 195 6.5.7 Device limitations and prospects . . . . . . . . . . . . . . . . 198 7 Conclusion and outlook . . . . . . . . . . . . . . . . . 207 7.1 Charge-carrier injection into doped films . . . . . . . . . . . . . . . 207 7.2 Charge-carrier transport in hot OLEDs . . . . . . . . . . . . . . . . 208 7.2.1 Prospects for OLED lighting facing tristable behavior . . . . 209 7.2.2 Outlook: Accessing the hidden PDR 2 region . . . . . . . . . 210 7.3 Charge-carrier recombination and spin mixing . . . . . . . . . . . . 211 7.3.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 7.3.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Bibliography. . . . . . . . . . . . . . . . . 215 Acknowledgements . . . . . . . . . . . . . . . . . 249Wer sich heute fĂŒr ein neues Mobiltelefon entscheidet, kauft damit wahrscheinlich einen Bildschirm aus organischen Leuchtdioden (OLEDs). Durch ihre brillanten Farben, die hohe erreichbare Auflösung und eine vergleichsweise einfache Prozessierung werden OLEDs als relativ neue Bildschirmtechnologie in den nĂ€chsten Jahren wohl den grĂ¶ĂŸten Marktanteil ausmachen. Da sie nicht auf der starren Kristallstruktur klassischer Halbleiter beruhen und als planare DĂŒnnschichtmodule produziert werden können, ermöglichen sie außerdem die Fertigung großer FlĂ€chenstrahler auf flexiblen Substraten – ein sehr attraktives Szenario fĂŒr zukĂŒnftige Beleuchtungssysteme. Trotz dieser vielversprechenden Eigenschaften steht der Durchbruch der OLED-Technologie als Leuchtmittel noch aus und erfordert weitere Forschung. Die Dynamik der LadungstrĂ€ger (Elektronen und Löcher) in einer OLED charakterisiert wichtige Teile der Bauteilfunktion und ermöglicht daher das VerstĂ€ndnis grundlegender physikalischer Konzepte und PhĂ€nomene. Diese Arbeit leitet anhand dieser Beschreibung experimentelle Methoden und Bauteilkonzepte ab, um die Effizienz und StabilitĂ€t von OLEDs zu optimieren. OLEDs zeichnen sich dadurch aus, dass ein elektrischer Strom aus LadungstrĂ€gern (Elektronen und Löchern) möglichst effizient unter Aussendung von Licht rekombiniert. Diesem Prozess geht eine LadungstrĂ€gerinjektion und deren Transport zur Emissionsschicht voraus. Diese drei Aspekte werden in dieser Arbeit zusammenhĂ€ngend diskutiert. Als erstes wird eine Methode vorgestellt, die InjektionswiderstĂ€nde anhand eines einfachen Experimentes quantifiziert. Sie bildet eine wertvolle Möglichkeit, Injektionsschichten besser zu verstehen und zu optimieren. Anschließend wird der LadungstrĂ€gertransport bei hohen elektrischen Strömen untersucht, wie sie fĂŒr OLEDs als helle Beleuchtungselemente nötig sind. Hier werden elektro-thermische Effekte vorgestellt, die physikalische Grenzen fĂŒr das Design und die Funktion von OLED Modulen bilden und deren plötzliches Versagen erklĂ€ren. Abschließend wird die Dynamik der stark elektrostatisch gebundenen LadungstrĂ€gerpaare, sogenannter Exzitonen, kurz vor deren Rekombination untersucht. Es werden verschiedene Möglichkeiten vorgestellt sie so zu manipulieren, dass sich das Abstrahlverhalten der OLED anhand bestimmter Anforderungen einstellen lĂ€sst.:List of publications . . . . . . . . . . . . . . . . . v List of abbreviations . . . . . . . . . . . . . . . . . ix 1 Introduction . . . . . . . . . . . . . . . . . 1 2 Fundamentals . . . . . . . . . . . . . . . . . 5 2.1 Light sources and the human society . . . . . . . . . . . . . . . . . 5 2.1.1 Human light perception . . . . . . . . . . . . . . . . . . . . 8 2.1.2 Physical light quantification . . . . . . . . . . . . . . . . . . 10 2.1.3 Non-visual light impact . . . . . . . . . . . . . . . . . . . . . 13 2.1.4 Implications for modern light sources . . . . . . . . . . . . . 15 2.2 Organic semiconductors . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2.1 Molecular energy states . . . . . . . . . . . . . . . . . . . . . 18 2.2.2 Intramolecular state transitions . . . . . . . . . . . . . . . . 24 2.2.3 Molecular films . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.2.4 Electrical doping . . . . . . . . . . . . . . . . . . . . . . . . 34 2.2.5 Charge-carrier transport . . . . . . . . . . . . . . . . . . . . 36 2.2.6 Exciton formation and recombination . . . . . . . . . . . . . 38 2.2.7 Exciton transfer . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.3 Organic light-emitting diodes . . . . . . . . . . . . . . . . . . . . . 44 2.3.1 Structure and operation principle . . . . . . . . . . . . . . . 44 2.3.2 Metal-semiconductor interfaces . . . . . . . . . . . . . . . . 47 2.3.3 Typical operation characteristics . . . . . . . . . . . . . . . . 49 2.4 Colloidal nanocrystal emitters . . . . . . . . . . . . . . . . . . . . . 52 2.4.1 Terminology: Nanocrystals and quantum dots . . . . . . . . 52 2.4.2 The particle-in-a-box model . . . . . . . . . . . . . . . . . . 54 2.4.3 Surface passivation . . . . . . . . . . . . . . . . . . . . . . . 55 3 Materials and methods . . . . . . . . . . . . . . . . . 57 3.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.1 OLED materials . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.2 Materials for photoluminescence . . . . . . . . . . . . . . . . 60 3.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.2.1 Thermal evaporation . . . . . . . . . . . . . . . . . . . . . . 62 3.2.2 Solution processing . . . . . . . . . . . . . . . . . . . . . . . 64 3.3 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.3.1 Absorbance spectroscopy . . . . . . . . . . . . . . . . . . . . 66 3.3.2 Photoluminescence quantum yield . . . . . . . . . . . . . . . 66 3.3.3 Excitation sources . . . . . . . . . . . . . . . . . . . . . . . 67 3.3.4 Sensitive EQE for absorber materials . . . . . . . . . . . . . 68 3.4 Exciton-lifetime analysis . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.1 Triplet lifetime . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.2 Singlet-state lifetime . . . . . . . . . . . . . . . . . . . . . . 70 3.4.3 Lifetime extraction . . . . . . . . . . . . . . . . . . . . . . . 70 3.5 OLED characterization . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.5.1 Current-voltage-luminance and quantum efficiency . . . . . . 73 3.5.2 Temperature-controlled evaluation . . . . . . . . . . . . . . . 74 4 Charge-carrier injection into doped organic films . . . . . . . . . . . . . . . . . 77 4.1 Ohmic injection contacts . . . . . . . . . . . . . . . . . . . . . . . . 79 4.2 Device architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.1 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.2 Device symmetry . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.3 Device homogeneity . . . . . . . . . . . . . . . . . . . . . . . 83 4.3 Resistance characteristics . . . . . . . . . . . . . . . . . . . . . . . . 84 4.3.1 Experimental results . . . . . . . . . . . . . . . . . . . . . . 84 4.3.2 Equivalent-circuit development . . . . . . . . . . . . . . . . 85 4.4 Impedance spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 92 4.4.1 Measurement fundamentals . . . . . . . . . . . . . . . . . . 92 4.4.2 Thickness dependence . . . . . . . . . . . . . . . . . . . . . 93 4.4.3 Temperature dependence . . . . . . . . . . . . . . . . . . . . 95 4.5 Depletion zone variation . . . . . . . . . . . . . . . . . . . . . . . . 97 4.6 Molybdenum oxide as a case study . . . . . . . . . . . . . . . . . . 99 5 Charge-carrier transport and self-heating in OLED lighting . . . . . . . . . . . . . . . . .101 5.1 Joule self-heating in OLEDs . . . . . . . . . . . . . . . . . . . . . . 104 5.1.1 Electrothermal feedback . . . . . . . . . . . . . . . . . . . . 104 5.1.2 Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 5.1.3 Cooling strategies . . . . . . . . . . . . . . . . . . . . . . . . 106 5.2 Self-heating causes lateral luminance inhomogeneities in OLEDs . . 108 5.2.1 The influence of transparent electrodes . . . . . . . . . . . . 108 5.2.2 Luminance inhomogeneities in large OLED panels . . . . . . 110 5.3 Electrothermal OLED models . . . . . . . . . . . . . . . . . . . . . 112 5.3.1 Perceiving an OLED as thermistor array . . . . . . . . . . . 112 5.3.2 The OLED as a single three-layer thermistor . . . . . . . . . 114 5.3.3 A numerical 3D model of heat and current flow . . . . . . . 116 5.4 OLED stack and experimental conception . . . . . . . . . . . . . . 118 5.5 The Switch-back effect in planar light sources . . . . . . . . . . . . 120 5.5.1 Predictions from numerical 3D modeling . . . . . . . . . . . 121 5.5.2 Experimental proof . . . . . . . . . . . . . . . . . . . . . . . 124 5.5.3 Variation of vertical heat flux . . . . . . . . . . . . . . . . . 127 5.5.4 Variation of the OLED area . . . . . . . . . . . . . . . . . . 131 5.6 Electrothermal tristabilities in OLEDs . . . . . . . . . . . . . . . . 133 5.6.1 Observing different burn-in schematics . . . . . . . . . . . . 133 5.6.2 Bistability and tristability in organic semiconductors . . . . 134 5.6.3 Experimental indications for attempted branch hopping . . . 138 5.6.4 Saving bright OLEDs from burning in . . . . . . . . . . . . 144 5.6.5 Taking another view onto the camera pictures . . . . . . . . 145 6 Charge-carrier recombination and exciton management . . . . . . . . . . . . . . . . .147 6.1 Optical down conversion . . . . . . . . . . . . . . . . . . . . . . . . 149 6.1.1 Spectral reshaping of visible OLEDs . . . . . . . . . . . . . 149 6.1.2 Infrared-emitting OLEDs . . . . . . . . . . . . . . . . . . . . 155 6.2 Dual-state Förster transfer . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.2 Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 6.3 Singlet fission and triplet fusion in rubrene . . . . . . . . . . . . . . 161 6.3.1 Photoluminescence of pure and doped rubrene films . . . . . 163 6.3.2 Electroluminescence transients of rubrene OLEDs . . . . . . 172 6.4 Charge transfer-state tuning by electric fields . . . . . . . . . . . . . 177 6.4.1 CT-state tuning via doping variation . . . . . . . . . . . . . 177 6.4.2 CT-state tuning via voltage . . . . . . . . . . . . . . . . . . 180 6.5 Excursus: Exciton-spin mixing for wavelength identification . . . . 183 6.5.1 Characteristics of the active film . . . . . . . . . . . . . . . . 184 6.5.2 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 6.5.3 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 6.5.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 6.5.5 Application demonstrations . . . . . . . . . . . . . . . . . . 192 6.5.6 All-organic device . . . . . . . . . . . . . . . . . . . . . . . . 195 6.5.7 Device limitations and prospects . . . . . . . . . . . . . . . . 198 7 Conclusion and outlook . . . . . . . . . . . . . . . . . 207 7.1 Charge-carrier injection into doped films . . . . . . . . . . . . . . . 207 7.2 Charge-carrier transport in hot OLEDs . . . . . . . . . . . . . . . . 208 7.2.1 Prospects for OLED lighting facing tristable behavior . . . . 209 7.2.2 Outlook: Accessing the hidden PDR 2 region . . . . . . . . . 210 7.3 Charge-carrier recombination and spin mixing . . . . . . . . . . . . 211 7.3.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 7.3.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Bibliography. . . . . . . . . . . . . . . . . 215 Acknowledgements . . . . . . . . . . . . . . . . . 24

    Fabrication and Measurement of LT-GaAs Photoconductive Antennas and Arrays

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    This thesis presents the fabrication and measurement of LT-GaAs based terahertz (THz) photo conductive antennas (PCAs) and arrays. The LT-GaAs THz PCAs are fabricated to serve as reference devices to new 2D material black phosphorous (BP) based THz PCAs. The LT-GaAs and BP devices have identical metallic electrodes, allowing for a comparison of emitted THz intensity and bandwidth. All PCAs have been measured using an open bench pulsed time-domain spectroscopy (TDS) system with a usable bandwidth from 0.1-4 THz, pumped with a 780nm Ti:Sapphire femtosecond laser. The results have shown LT-GaAs devices outperforming BP devices in signal amplitude and bandwidth at identical DC bias voltages and pump powers. Three other electrode shape designs were achieved: circular, slotted, and fractal, fabricated on LT-GaAs. The effect of electrode shape on the amplitude and bandwidth of the THz pulse has been experimentally characterized. A comparison of all four shapes has shown that the bowtie electrodes provide a nominal increase in pulse amplitude under identical biasing conditions. Further, a polarization study using an x-cut quartz rotator was conducted, validating that all four electrode shapes are highly linearly polarized. Results show that the co-polarized THz pulse is two orders of magnitude greater than the cross-polarized THz pulse. In addition, two element THz PCAs with electrode spacings of 75”m, 150”m, and 300”m have been investigated. A novel feed network using two beam splitters has been designed, and implemented into the existing open bench TDS system. This feed network gives individual control over the position and path length of the beams feeding each elements. Further, a DC bias splitting PCB and switchboard were designed to allow each element to be turned on and off, aiding in laser alignment validation. The measurement of all three devices have shown the array THz pulse having a higher amplitude than either individual element, however, an insignificant effect on the array bandwidth has been observed

    Fabrication and Measurement of LT-GaAs Photoconductive Antennas and Arrays

    Get PDF
    This thesis presents the fabrication and measurement of LT-GaAs based terahertz (THz) photo conductive antennas (PCAs) and arrays. The LT-GaAs THz PCAs are fabricated to serve as reference devices to new 2D material black phosphorous (BP) based THz PCAs. The LT-GaAs and BP devices have identical metallic electrodes, allowing for a comparison of emitted THz intensity and bandwidth. All PCAs have been measured using an open bench pulsed time-domain spectroscopy (TDS) system with a usable bandwidth from 0.1-4 THz, pumped with a 780nm Ti:Sapphire femtosecond laser. The results have shown LT-GaAs devices outperforming BP devices in signal amplitude and bandwidth at identical DC bias voltages and pump powers. Three other electrode shape designs were achieved: circular, slotted, and fractal, fabricated on LT-GaAs. The effect of electrode shape on the amplitude and bandwidth of the THz pulse has been experimentally characterized. A comparison of all four shapes has shown that the bowtie electrodes provide a nominal increase in pulse amplitude under identical biasing conditions. Further, a polarization study using an x-cut quartz rotator was conducted, validating that all four electrode shapes are highly linearly polarized. Results show that the co-polarized THz pulse is two orders of magnitude greater than the cross-polarized THz pulse. In addition, two element THz PCAs with electrode spacings of 75”m, 150”m, and 300”m have been investigated. A novel feed network using two beam splitters has been designed, and implemented into the existing open bench TDS system. This feed network gives individual control over the position and path length of the beams feeding each elements. Further, a DC bias splitting PCB and switchboard were designed to allow each element to be turned on and off, aiding in laser alignment validation. The measurement of all three devices have shown the array THz pulse having a higher amplitude than either individual element, however, an insignificant effect on the array bandwidth has been observed

    Integrated widely tunable laser systems at 1300 and 1550 nm as swept sources for optical coherence tomography

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    Engineering of reconfigurable integrated photonics for quantum computation protocols

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    Over the last decade, integrated optics has emerged as one of the main technologies for quantum optics and more generally quantum computation, quantum cryptography and communication. In particular, it is fundamental for the construction of reconfigurable interferometers with a high number of optical modes. In this thesis we present, on the one hand, the development of a new geometry for the creation of integrated reconfigurable devices with a high number of modes and, on the other hand, the development of quantum computation protocols to be realized in integrated photonic chips. In the first part, two algorithms are proposed for the characterization of integrated circuits in terms of implemented unitary matrix. The first uses a so-called Black Box approach, i.e. one that makes no assumptions about the internal structure of the device under consideration, and it is based on second-order correlation measurements with coherent light. The second is specific to a planar rectangular geometry, first proposed by Clements et al., which has a variety of applications in the literature and is also employed in this thesis. Subsequently, we present the realization of a new 32-mode reconfigurable integrated photonic device with a continuously coupled three-dimensional geometry. Its potential in terms of reconfigurability is tested and a Boson sampling experiment with three and four photons is carried out to show its potential in the field of quantum computation. In the second part, we propose the application of integrated photonic devices to two quantum computation protocols. The first was recently proposed and is the quantum extension of a problem called Bernoulli factory. It consists in the construction of a qubit from nn qubits in the same unknown state so that there is a predetermined exact relation between the output and input states. In the thesis, we theoretically analyze the computational complexity of the problem in terms of the qubits used and the success probability of the problem. Furthermore, a photonic implementation is proposed and experimentally tested for correctness and resilience to experimental noise. The second application consists of the experimental implementation of a quantum metrology protocol in which three distinct phases are estimated simultaneously, showing that the use of indistinguishable photons leads to an advantage in terms of the variance of the estimates
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